In recent years the availability and quality of UV radiation data have improved appreciably. However, high quality measurements from intercalibrated ground-based spectrometers are available at only a few sites worldwide. Satellite derived UV irradiances, on the other hand, offer the advantages of global coverage and avoid the problems of instrument intercalibrations when investigating geographic differences in UV. However, with the satellite sensors in common use, the retrievals can be subject to errors due to variability in the distribution of ozone and aerosols in the lower atmosphere. Previous studies have shown good agreement between satellite-derived UV and ground-based measurements at pristine locations, but with increasing overestimations of the surface UV at more polluted sites, which are characterized by larger concentrations of ozone and aerosols in the troposphere. Consequently, the contrast in UV between pristine locations and more polluted locations may be underestimated by the satellite retrievals. Here we investigate the relative contribution to these differences due ozone profile differences and aerosol extinction. It is found that both factors contribute to the errors, but that the aerosol effect dominates. Tropospheric aerosols can result in satellite overestimations exceeding 30% in populated regions. Large concentrations of tropospheric ozone also lead to satellite overestimations. Firstly, the total ozone column would be underestimated, and secondly, the underestimated component is disproportionately important because of the increased path length and warmer temperatures in the troposphere. If tropospheric ozone were less than expected, as in more pristine locations, then there would be a tendency for satellite-derived UV to be too large by up to ~ 5%.

One of the key components in understanding how global climate change effects the biological environment on Earth is determining how the amount of UV radiation reaching the surface has changed in response. In order to accurately quantify such changes, the role of tropospheric aerosols in both changing the amount of UV surface radiation and affecting the measurement of such radiation must be better understood. In this paper we will provide an overview on our current understanding of troposheric aerosols and their effect on UV radiation. We will also describe how such aerosols are taken into account when trying to measure the UV surface radiation, and we will show how the diurnal variability of tropospheric aerosols lead to large errors in determining UV surface radiation from space measurements. Finally, we will describe how measurement from future sensors launched into orbit other than those low to the Earth can reduce such errors.

The new Version 8 TOMS 360 nm reflectivity time series 1980 to 1992 and 1997 to 2001 have been combined to estimate changes that have occurred over a 21-year period. The observed changes are mostly related to changes in cloud cover and aerosols, since the Earthy appears dark (2 to 6% reflectivity) at 360 nm. The relative radiance calibration of the two TOMS (Nimbus-7, N7 and Earth-Probe, EP) has been adjusted using the measured minimum reflectivity over mid-latitude ocean and land locations (±50°). The result is that the previously published N7 minimum reflectivity decreased by 0.02 and now matches the better-calibrated Earth-Probe/TOMS. Most of the local trend features seen in the N7 time series (1980 to 1992) have been continued in the combined time series, but the overall zonal average and global trends have changed. The correlation of cloud cover with solar activity (measured by the 10.7 cm solar radiation) that was present during the Nimbus-7 period (1980 to 1992) is no longer evident for the period 1980 to 2001. The key results include a continuing decrease in cloud cover over Europe and North America and an increase in reflectivity near Antarctica.

Actinic flux (the radiation incident on the surface of a small sphere) is the measure of radiation required for atmospheric chemistry. The photolysis rate for a given species is the product of its absorption cross-section, quantum yield and the actinic flux, integrated over all relevant wavelengths. Despite their importance, actinic flux and photolysis rate data are not routinely available since radiation monitoring sites usually report irradiances (radiation on a flat, horizontal surface). Using data from Reading (UK) we show that spectral UV actinic fluxes can be derived from spectral irradiance measurements with a scanning spectroradiometer (Bentham DTM300) to within an uncertainty of 20% full range, or 15% if sky conditions are known. Any photolysis rate dependent on these wavelengths can then be calculated from the actinic flux data. The disadvantage of spectroradiometers is their scan time of several minutes. The GUV-541 multi-band radiometer measures across 5 narrow UV channels simultaneously and samples at a rate of 2-3Hz. Using a semi-empirical formula, this multi-band irradiance data can be converted into photolysis rates for J(O₃) and J(NO₂). The GUV derived photolysis rates were within 20% of those calculated from spectral actinic fluxes under all conditions tested. This compares well with other methods of measuring photolysis rates.

Theoretical and experimental investigations of the effects of atmospheric aerosols on diffuse sky radiance in the visible and infrared range of the solar spectrum show that the retrieval of physical aerosol properties from intensity measurements of diffuse skylight leads to non-unique solutions for aerosol optical depth, complex refraction index and aerosol size distribution. Additional photopolarimetric radiance measurements have shown to add valuable information to intensity data, thus allowing a more specific determination of aerosol parameters. The extension of these retrieval algorithms to the UV range provides additional information, but requires the development of sophisticated radiative transfer models which account for polarization effects on molecular and aerosol scattering as well as for multiple scattering processes in the earth’s atmosphere. In order to provide a reference for these models, radiance measurements in the UV and visible range of the solar spectrum have been performed with a high resolution Bentham DTM300 double monochromator, equipped with a linear sheet polarizer. The measurements show strong differences between the directional distributions of horizontally and vertically polarized diffuse sky radiance of the upper hemisphere. Comparison between data taken at the Jungfraujoch (Switzerland, 3576 m a.s.l.) and in Thessaloniki (Greece, 20 m a.s.l.) under different atmospheric aerosol conditions reveals that aerosol Mie scattering effects horizontally and vertically polarized radiance in different ways, which confirms that photopolarimetric radiance data contain more information about aerosol properties than intensity measurements alone.

Impacts of UV radiation on humans, plants and animals involve both acute exposures to high levels of UV radiation and chronic exposures to moderate to high levels of UV radiation. However exposure estimates are often limited by the relatively low density of ground-based measurements of solar ultraviolet irradiance. This study relates the spacing of measurements to the accuracy of estimating daily exposure (chronic exposure) and maximum daily exposure over a 7-day interval (acute exposure) to solar UV. 300 nm, 368 nm, and UVB broadband measurements using multi-filter rotating shadow band radiometers and a broadband radiometer were paired by day for locations within a climate region. The variability in exposures in the various wavebands was evaluated using spatial statistics and interpolations made using kriging methods. The spatial correlation of the 300 nm and UVB acute exposures were greater than the correlation for the chronic daily exposures for distances up to 500 km. Conversely, the spatial correlation of the 368 nm acute exposure was greater than that for the chronic daily exposures for distances up to 500 km. A decrease in the variability in the acute over chronic exposure resulted in an improved ability to estimate the acute exposure across the full 1100 km domain. Limitations to the interpolation of 368 nm acute exposure were assumed to be due to local cloud effects on the radiation. Improvements in the ability to interpolate chronic 300 nm, 368 nm, and UVB exposures from the present USDA UVB Climate Network will require longer exposure periods.

Ultraviolet radiation from the sun, especially the UVB (280 to 320 nm), has important roles in urban ecosystems, including effects on human health. Broadband UVB radiation is being continuously monitored in the city of Baltimore, MD as part of a long-term ecological research program, the Baltimore Ecosystem Study. This paper compares above-canopy broadband UVB irradiance at the Baltimore station to broadband UVB irradiance at a more-rural station 64 km SE (at Wye Research Center in Queenstown, MD) and a station characterized as suburban within the Baltimore-Washington metropolitan area, 42 km SW (at Beltsville Agricultural Experiment Station). The Baltimore data are from the initial 14 months of measurements there. The solar radiation monitoring station in Baltimore is located on a 33-m-tall building on a high point with no significant obstructions to sky view. The broadband instruments, all of which were provided by the USDA UVB Monitoring and Research Program, were calibrated in the same facility, the NOAA Central UV Calibration Facility in Colorado. In general, UVB irradiances at the three sites were similar. Over all conditions, Baltimore and the suburban site measured 3.4% less irradiance than the rural site. This difference is within the anticipated ±3% calibration uncertainty of the broadband pyranometers. On the 59 days with cloud-free conditions at all three sites, the average differences between measured UVB at the three sites was even smaller; Baltimore measured 1.2% less irradiance than the rural site. On the clear days, differences between total daily irradiance and the trend of daily irradiance through the year were clearly related to total column ozone as indicated by the EPTOMS satellite. High aerosol optical thickness strongly reduced daily UVB dose; whereas [SO₂] had no influence. Surface O₃ increased with increasing UVB dose when [NO₂] exceeded 10 ppb.

Most comparisons of TOMS estimates of surface UV irradiation with measured values from ground-based instruments have indicated a bias of the TOMS estimates toward larger values. A portion of this bias results from absolute uncertainties in the ground-based instruments. The comparison reported here is based on ground-based data from four sites in the UGA/EPA Brewer network. The raw data from the ground-based instruments has been corrected for (1) stray light rejection, (2) the cosine errors associated with the full sky diffuser, (3) the temperature dependence of the response of the instruments and (4) the temporal variation in the instrument response reducing the estimated errors of the absolute irradiance values of each spectral measurement to < ±7%. Comparisons of TOMS with the surface measurements are performed both at spectrally resolved wavelengths at the time of overpass and for erythemally-weighted daily-integrated doses. These comparisons are made for all days and for clear-sky days only. The comparisons are carried out using both linear regressions of scatter plots of the two sets of data and for mean differences with respect to both TOMS and the Brewer measurements. It is found that spectrally resolved comparisons suffer from inconsistencies at some of the sites that are believed to result from wavelength uncertainties in the Brewer; they are therefore of more limited use than wavelength integrated data. A comparison based on daily-integrated doses shows only a small positive TOMS bias (4%) for clear-sky days with a somewhat larger bias (8%) for data taken from all days.

The U.S. National Science Foundation’s network for monitoring UV radiation in polar regions is now in its 15^th year of operation. During this period, the deployed SUV-100 spectroradiometers have repeatedly been modified, and data processing methods have been changed. These modifications have continuously improved the quality of published data, but have also introduced step-changes into the data set. For example, a change of the wavelength calibration method in 1997 has improved the wavelength accuracy to ±0.04 nm (±1σ), but also lead to a step of 2-4% in published biological dose rates. In order to best assess long-term changes in UV at network locations, it is desirable to remove these steps and to homogenize the data set. This publication discusses possible ways to accomplish these objectives, with special emphasis on absolute calibration, wavelength accuracy, and the cosine error. To date, published data are not corrected for the instruments’ cosine errors. Such corrections are not straightforward, as older data are affected by an azimuth asymmetry of the irradiance collector, which was not constant over the years. A new method to correct the errors for both clear and cloudy sky conditions was developed, and is described here. Results indicate that dose rates published prior to the year 2000 are low by 2-5%, and exhibit a variation with the Sun’s azimuth angle. By modifying the instruments’ irradiance collectors in 2000, the azimuth asymmetry was virtually eliminated, however, the modification also lead to a step-change of about 3% in published data. The ability of the new correction algorithm to remove this step is demonstrated. Uncertainties in biologically weighted dose rates caused by the cosine error can be reduced with the correction procedure to ±2%. We are planning to reprocess the entire NSF data set with the new algorithms to improve both accuracy and homogeneity.

The authors examined the temporal and spatial variation of UV-B erythemal irradiance under cloudy stratocumulus conditions. A three-dimensional cloud model was constructed which had fractal properties in the horizontal. Fractal parameters were derived from 98 aircraft measurements in northwest Tasmania. Modelled radiation was compared with spatial sky radiance measurements using a Variable Sky Platform. Temporal measurements of irradiance were also compared with model predictions. In both areas both model and measurements agree well.

The IAI Network for the measurement of ultraviolet radiation in Chile, Argentina and Puerto Rico is composed of ten multi-channel radiometers (GUV 511, Bisopherical Instruments Inc.), which are periodically sun calibrated with a traveling reference GUV (RGUV). The RGUV is calibrated under solar light against a SUV100 spectroradiometer. This calibration is then transferred to each instrument in the network through the RGUV. A previous multi-regression model proved to be suitable to derive narrowband irradiance from broadband irradiance, ozone column and solar zenith angles (SZA). A recent modification of the existing multi-regression model improved the multi-channel instrument sun calibration against spectroradiometers. In this approach, the narrowband irradiance is the SUV spectral irradiance and the broadband is the multi-channel GUV irradiance. We included the azimuth angle as a parameter into the multi-regression equation and we applied a non-linear function, instead of a single coefficient, to correct for SZA. In this paper, the new multi-regression approach is applied to both steps of a GUV calibration: SUV - RGUV and RGUV - GUV and the results are compared with traditional calibration methods. Important improvements are observed in the calibration, in particular for SZA larger than 50°.

The potential for increased ultraviolet (UV) exposure resulting from ozone depletion has helped fuel interest in acquiring accurate in-water measurements of UV irradiance. In the water column, the flux in the UV becomes a vanishingly small signal that must be measured in the presence of a much larger visible component. In addition, the flux of UV in natural waters may be influenced greatly by changes in solar elevation and from focusing/defocusing by surface waves. For these reasons, an instrument that accurately measures the flux of UV in air will not work as well when submerged, and a number of elements must be optimized to produce instruments for use underwater. In response, Biospherical Instruments Inc. has produced the PUV-2500 Profiling Ultraviolet Radiometer. The system is designed to collect time series or vertical profiles of UV (305, 313, 320, 340, 380 and 395nm) and Photosynthetically Active Radiation (PAR: broadband 400-700 nm) irradiance underwater to depths of 350 meters. The PUV-2500 is designed to measure downwelling irradiance with a response capability exceeding nine decades of dynamic range, as well as pressure/depth, and water temperature. The PUV-2510 is a radiometrically-matching surface reference radiometer. Highly versatile because of its small size and lightweight design, the PUV-2500 can be used in traditional vertical profiling mode (surrounded by a lowering frame and supported by its own cable) or, by employing our free-fall design option, in free-descent mode, thereby helping to avoid artifacts associated with ship shadows.

Increased concentration of leaf UV-B absorbing-compounds due to exposure to UV-B radiation is widely accepted as one of the plant adaptations to resist enhanced UV-B radiation. This paper reports a field comparative study of dynamics and temporal changes of UV-B absorbing-compound concentration in 35 southern broadleaf trees over a growing season. Leaf UV-B absorbing-compound, chlorophyll concentration, and leaf thickness were measured from the sun-exposed leaves of 35 tree species collected monthly from individual trees growing within the city of Baton Rouge, Louisiana from April to October in 2000. The USDA UV-B Monitoring Network Baton Rouge Station provided the ambient UV-B radiation data. Leaf UV-B absorbing-compound concentration varied significantly with leaf age and species. Intra-specifically, leaf UV-B absorbing-compound concentration exhibited a generally increasing trend during leaf growth and development in response to the increased exposure to natural UV-B/solar radiation during the growing season. Inter-specifically, significant differences existed in leaf UV-B absorbing-compound concentration. The species were compared and ranked based on the growing season averages of the leaf total UV-B absorbing-compound concentration. The species were further classified into three levels (high, medium, and low) based on the magnitude of UV-B absorbing-compound concentration.

A decrease in stratospheric ozone would lead to increases in Ultraviolet-B irradiances reaching the earth’s surface. The effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, decrease in photosynthetic activity, susceptibility to disease, and changes in plant structure and pigmentation. A substantial number of studies have been conducted that have evaluated the potential consequences of an increase in UV-B radiation on many plants, but there are few studies that consider the changes of plant growth curve under increased UV-B radiation. This study evaluated availability of the four existing plant growth models under ambient level of UV-B radiation and supplemental UV-B radiation for soybean and winter wheat plants. Both of the Monomolecular and Divided simulation models proved not suitable for use in simulating plant growth under supplemental UV-B radiation. The Logistic model can be used to simulate plant growth in early growing stages and in the condition of Ck and T1 while the Gompertz model simulates plant growth better under T2 and T3 for soybean crop. For winter wheat crop, both of the Logistic and Gompertz models can be used to simulate plant growth under supplemental UV-B treatments.

While many studies have evaluated the chronic effects of exposure to enhanced levels of UV-B radiation on plants very few studies have evaluated the implications of plant development within a background of fluctuating levels of UV-B radiation. Much interest and concern surround the issue of stratospheric ozone depletion and concurrent increases in UV-B radiation and this remains a concern. However, variation in UV-B levels on a daily basis is largely due to cloud cover and tropospheric air quality as well as possible effects of fluctuations in the total ozone column. Therefore the importance of the effects of short-term changes in UV-B radiation is not predicated on the assumption of continued ozone destruction. In this study we evaluated to change in foliar phenolic composition in barley and the consequences of changes in these putative protection compounds on subsequent sensitivity to UV-V radiation. The UV-B exposure levels ranges from less than 1 to nearly 8 kJ m^-2 of biologically weighted UV-B radiation. Barley plants that developed under height ambient levels of UV-B radiation had higher levels of phenolics than control plants grown under the same conditions except with UV-B excluded. Those plants with higher phenolic content show some degree of increased protection from subsequent levels of UV-B as evidenced by less damage to DNA. However, it was also found that other environmental factors contributed to the induction of foliar screening compounds.

Solar ultraviolet radiation (UVR) gives rise to beneficial or adverse health effects depending on the dose. Excessive UV exposures are associated with acute and chronic health effect but in appropriate doses UV sunlight is advisable. Important biological function of UVR is initiation of endogenous synthesis of vitamin D in human skin. A useful method based on an in vitro model of vitamin D synthesis ('D-dosimeter') has been specially developed to measure the vitamin D synthetic capacity of sunlight in situ. For the first time laboratory and field tests have been performed to link commonly used erythemal units (MEDs) and previtamin D accumulation.

The determination of UV- and light doses received by people as a function of their activities and their environment, for present and future conditions, is the aim of the presented study. In this paper we present first preliminary results. Measurements of the total daily UV dose received by horizontal and vertical parts of the human body were performed on three chosen days in the region of Vienna, Austria. The measurements were performed in the UV and in the visible spectral range using ultraviolet selective sensors and sensors adapted to human eye sensitivity. Data acquisition was performed by using dataloggers. In this way it was also possible to determine the UV intensity and dose as a function of time and location. The UV intensity was determined for typical outdoor and indoor activities such as walking in a street, in a forest or in flat unobstructed areas. Indoors the determination of UV doses is more straightforward, the determination of the visible dose is however much more complex. A software was developed to determine the total daily dose received by the human body as a function of day and occupation.

Stratospheric ozone depletion has caused an increase in the amount of UV-B radiation reaching the earth’s surface. Numerous investigations have demonstrated that the effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, susceptibility to disease, and changes in plant structure and pigmentation. Many experiments examining UV-B radiation effects on plants were conducted in growth chambers or greenhouses. It has been questioned if the effect of UV-B radiation on plants can be extrapolated to field responses from indoor studies because of the unnaturally high ratios of UV-B/UV-A and UV-B/PAR in many indoor studies. Field studies on UV-B radiation effect on plants has been recommended in order to use the UV and PAR irradiance provided by natural light. This study reported the growth and yield responses of a maize crop exposed to enhanced UV-B radiation and the UV-B effects on maize seed qualities under field conditions. Enhanced UV-B radiation caused a significant reduction of the dry matter accumulation, and the maize yield in turn was affected. With increased UV-B radiation the flavonoid accumulation in maize leaves increased, and the contents of chlorophyll a, b, and (a+b) of maize leaves were reduced. The levels of protein, sugar, and starch of maize seed decreased with enhanced UV-B radiation, while the level of lysine increased with enhanced UV-B radiation.

This article analyzed annual, seasonal and daily variations of total solar and ultraviolet (UV) radiation, based on observed data over Nanjing area from May 2001 to April 2002. The study shows that the daily variation of solar radiation and UV-radiation is bigger at noon, smaller in the morning and afternoon. When looking at it on an annual scale the variation is bigger in summer, smaller in winter. A linear regression equation has been set up to calculate quantities of UV-radiation that reaches the earth in a sunny day from daily-observed data.

Ground-based measurement of ultraviolet irradiances with UV-A and UV-B sensors was made at Kwangju (35.13°N, 126.53°E), Korea from June 1998 to June 2001. These sensors measure integrated global UV radiation in 280-315 nm and 315-400 nm wavelength range, respectively. Measurement outputs from the UV sensors were sampled at every 15 seconds interval and saved every minute as a mean value into a data logger. The results showed that monthly mean values of UV-A and UV-B irradiances were maximized at 75 W/m² and 1.7 W/m², respectively. Total atmospheric optical depth was determined by analyzing the total and diffuse solar irradiances measured by a collocated multifilter rotating shadowband radiometer. Aerosol optical depth for 415 nm was determined by subtracting the contribution by Rayleigh scattering from total optical depth. Hourly and daily mean values of ground-based UV-A and UV-B irradiances were analyzed to investigate the effects of total atmospheric aerosol amount on them, especially those of the Asian dust storm particles in the spring.

Using an ultraviolet limb-scan technique, this paper carries out the feasibility study of joint retrieval of ozone and air density vertical distributions in the mesosphere and upper stratosphere. Ozone and air density profiles are important parameters for middle atmosphere circulation. Satellite observation is the major method to obtain the parameters globally. In past years, there have been many research satellites, such as SME, the Shuttle Flight STS-87, using UV and IR waveband instruments to retrieve these parameters by limb-scan technique. As UV radiances observed by the satellite contain both information of ozone content (via absorption) and air density (via Rayleigh scattering), it is feasible to retrieve simultaneously these two parameters. In this paper, based on a radiative transfer algorithm for limb-scan mode, the joint retrieval feasibility study is carried out. In this paper, the method based on the Direct Method proposed (1976, 1982) by Aruga et al. is applied to retrieve ozone and air density profiles using simulated satellite measured data of multiple ultraviolet wavelengths by limb scanning and the sensitivity tests of air density errors to ozone inversion precision are performed together. Here, a joint retrieval scheme of ozone and air density is established, which makes it possible to obtain air density profile ,as well as ozone profile with an improved accuracy in the mesosphere and upper stratosphere.

A Long Path Differential Optical Absorption Spectrometer (LP-DOAS) system has been developed to measure simultaneously atmospheric trace gases concentration and spectral extinction coefficient in the visible region over a few hundred meter. Concentrations of atmospheric gases; SO₂, NO₂, and O₃ are evaluated from the differential optical absorption spectra measured by system. Performance of the system was tested in the field over a 1500 m path length. The Differential Optical The field measurements were carried out and compared with those of a fixed monitoring system. The results show very good correlation, R² > 0.7, for two gas species: NO₂, and O₃. Atmospheric visibility is estimated by integrating the measured spectral extinction coefficient over the visible region, at 550 ± 3 nm by considering the stability of xenon lamp and transmitted light intensity. During the measurement, average exctinction is 0.51/km and average visual range is 10.1 km. The amount of PM10 and the relative humidity affect the extinction. Because of the alignment of the optic system in DOAS system all measured data did not have quantitatively significant extinction coefficients.

The background and an approach for deriving tropospheric ozone, water vapor and aerosols from direct sun observations, zenith sky radiances and the ratio of radiances at large zenith (near horizon) to the corresponding zenith sky radiances is described. Surface based remote sensing measurements of zenith sky and large zenith angle atmospheric radiance measurements have been made in the wavelength range of 305 nm to 923 nm near local noon from a mountain site at an altitude of 2.6 km in the vicinity of Boulder, Colorado. The wavelength dependence of the limb/zenith radiance ratios indicate significant limb darkening in the ultraviolet that changes to limb brightening with increasing wavelengths that is a maximum in the blue. Model calculations of a molecular atmosphere with absorption by ozone for wavelengths between 300 and 400 nm indicates that this ratio is sensitive to tropospheric ozone. Results from measurements and model calculations of the sensitivity of these ratios to tropospheric ozone are presented.

The changes in growth, gas exchange, and stable carbon isotope value (δ¹³C) in soybean seedlings (Glycine max L. Merr. cv. Longdou No.1) exposed to a 11.2 kJ m^-2 day^-1 biologically effective UV-B radiation (UV-B_BE, 280-315 nm) and 50 mg CdCl₂ kg^-1 vermicular treatment, either alone or in combination were investigated under greenhouse conditions. Compared to the control, plant height and biomass, photosynthesis rates (Pn), water-use efficiency (WUE), and stomatal conductance (Gs) decreased significantly (P<0.05) exposed to cadmium (Cd) or UV-B radiation and combination. Leaf stable carbon isotope composition (δ¹³C values) decreased by Cd or UV-B radiation alone and in combination, δ¹³C were significantly correlated with changes in Pn, WUE and biomass. But Cd and UV-B in combination did not cause greater changes compared to Cd or UV-B radiation alone in many parameters. These results suggested that there was relatively little interaction of the two stresses. δ¹³C values may provide a reliable indication for growth, WUE, Pn or biomass of soybean seedlings when exposed to UV-B radiation and cadmium pollution.

The greater UV-B radiation has heightened concern that it has adverse impact on agricultural production and natural plant ecosystems. The effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, decrease in photosynthetic activity, susceptibility to disease, and changes in plant structure and pigmentation. Plant responses to UV-B enhancements also include increased accumulation of flavoids, increased leaf thickness, increased reflectance of leaves, reductions in growth, and direct damage to photosynthetic mechanisms. One-third to one-half of all plant species tested are deleteriously affected by UV-B irradiance levels “above ambient.” This experiment was conducted to study responses of winter wheat growth and production under both ambient and sub-ambient UV-B irradiance (15.3% UV-B irradiance decrease from ambient level). Sub-ambient UV-B irradiance increased winter wheat growth and production, which indicates that current levels of ambient UV-B irradiance might have an adverse effect in tested areas.

Cucumber plants (Cucumis sativus L. cv. Jinchun No 3) grown in a greenhouse were treated with three different biologically effective ultraviolet-B (UV-B) radiation levels: 1.28 kJ. m^-2 (CK), 8.82kJ.m^-2 (T1) and 12.6 kJ. m^-2 (T2). Irradiances corresponded to 8% and 21% reduction in stratospheric ozone in Lanzhou. Plants at three-leaf stage were irradiated 7 h daily for 25 days. The growth, stomata, flavonoid and ABA content in cucumber leaves exposed to 3 levels of UV-B radiation were determined in this paper. The results indicated that, compared with the control after 25 days UV-B radiation, RI of cucumber under T1 treatment is -18.0% and RI under T2 treatment is -48% mostly because of the reduce of leave area and dry weight accompanying with the increase of SLW; the rate of stomata closure under the treatments of T1 and T2 on the 6th day was up to respectively 70% and 89%, and amounted to 90% and 100% on the 18th day, and the guard cells in some stomata apparatus became permanent pores and lost their function at the same time; with the duration of UV-B radiation, the rise of the absorbance to ultraviolet light (305nm) showed the content increase of flavonoid; Abscisic acid (ABA) was determined by means of ELISA which showed that under the T1 treatment, the content of ABA was up to maximum to 510% higher than that of the control on the 21st day, meanwhile, under the treatment of T2, it was the highest on the 18th day to 680% of the control, and then had a decrease tendency on 21st day. The result still indicated that ABA accumulation could be induced by enhanced UV-B the radiation. The bigger was the dose of radiation, the higher was the accumulation of ABA. When intensity of UV-B radiation went beyond the degree of endurance of cucumber plants, ABA content descended then. Cucumber plants resist enhanced UV-B radiation by means of improving the contents of ABA and flavonoid. The increase of ABA content in cucumber leaves could lead to the stomata closure. Therefore, the changes of ABA content and absorbance, the rate of stoma closure in cucumber leaves were the adaptive mechanism to enhanced UV-B radiation.

QASUME is a European Commission funded project that aims to develop and test a transportable unit for providing quality assurance to UV spectroradiometric measurements conducted in Europe. The comparisons will be performed at the home sites of the instruments, thus avoiding the risk of transporting instruments to participate in intercomparison campaigns. Spectral measurements obtained at each of the stations will be compared, following detailed and objective comparison protocols, against collocated measurements performed by a thoroughly tested and validated travelling unit. The transportable unit comprises a spectroradiometer, its calibrator with a set of calibration lamps traceable to the sources of different Standards Laboratories, and devices for determining the slit function and the angular response of the local spectroradiometers. The unit will be transported by road to about 25 UV stations over a period of about two years. The spectroradiometer of the transportable unit is compared in an intercomparison campaign with six instruments to establish a relation, which would then be used as a reference for its calibration over the period of its regular operation at the European stations. Different weather patterns (from clear skies to heavy rain) were present during the campaign, allowing the performance of the spectroradiometers to be evaluated under unfavourable conditions (as may be experienced at home sites) as well as the more desirable dry conditions. Measurements in the laboratory revealed that the calibration standards of the spectroradiometers differ by up to 10%. The evaluation is completed through comparisons with the same six instruments at their homes sites.

The seasonal variation of the surface albedo, due to snow or ice, complicates satellite estimation of the high-latitude surface UV irradiance. The TOMS instrument, that measures the backscattered radiances from the Earth's atmosphere and surface, does not distinguish cloud backscattering from surface backscattering. When the TOMS UV algorithm is used, false interpretation of the measured high reflectivity as thick cloudiness leads to substantial underestimation of the surface UV irradiance. While the largest UV irradiance is usually received during the summer, the spring exposure to UV radiation is the main concern in high-latitudes since the sensitivity of some biological organisms to UV radiation is more pronounced at low temperatures, and snowcover enhances the surface UV irradiance. This paper presents a new method for estimation of the surface reflectivity. The method is based on analysis of the TOMS Lambertian equivalent reflectivity data using the moving time-window technique. The new method treats the measured reflectivity data as samples from a distribution whose lower tail corresponds to surface albedo. The basic method assumes that the distribution is homogeneous, i.e. the surface albedo is constant within the window. Adequate statistics is achieved only by using a wide time-window which, unfortunately, leads to underestimation of the surface albedo during spring and autumn transitions. Therefore, the method was developed further to account for transitions. The feasibility of the new method has been studied globally for high-latitude regions, and it is expected to improve springtime UV irradiance estimates of polar regions.